The frequency of the correction in the HK 990 room-correction system can be limited to 1 kHz corrected. 1 kHz is slightly above the room-dominated region (500Hz). Full-band correction (to 20kHz) is also an option. The system not only makes the standard set of measurements at the listener's seat but also require you to position the microphone two feet away from the speaker, on axis, to estimate the direct near-field response without room reflections. Not surprisingly, the accuracy of the measurements two feet away can be problematic because two feet is inadequate for the wave front of some large complex (4 way for example) speakers to converge. Harman engineers indicated that the microphone could be moved back to three feet or even half the distance between the speaker to the listening seat to handle this issue.

The near-field measurement is a unique feature of the HK 990. One assumes the room-correction system uses the near-field measurements as a guide for the EQ above 1 kHz. Above 1 kHz, a speaker's near-field frequency response errors dominate over room related effects. Unfortunately, it is not as simple as making a direct near field measurement. The anechoic off-axis response of the speaker also affects what the listener hears. Harman may have guessed the end user has limited patience for making measurements at each speaker.

The HK 990 room-correction system supports two mono subwoofers with a separate room correction filter in each signal path. Harman Labs developed the multiple mono subwoofer technique to reduce variation of the frequency response at the listeners' seats. The lab identified four as the optimal number:

The four subwoofer technique also widens the room area where the response of the subwoofer is well controlled. A home theater with six to nine seats is the obvious application for the technique. The full four-channel subwoofer system can be implemented with a product called BassQ. It is an external box that corrects only the subwoofers and is marketed under the JBL Synthesis brand to custom installers, though is made available on request to consumers. The HK 990 and AVR 7550HD use this technique with a two subwoofer limit. Only with two subwoofers does the HK 990 room-correction system allow multiple in-room microphone measurements. A single room measurement is made at the prime listening position if the speakers are run full range or with one subwoofer.

The HK 990 cannot be connected to a computer to display pre- and post-corrected frequency response on a computer screen. This is a feature on some competitive systems. Disappointingly, a calibrated microphone is also not provided. By "calibrated," I mean the microphone was placed in a test device with a reference microphone known to be flat. A file is created that represents the microphone error. The microphone is marked with a number that matches the file. When you set up a room EQ with a calibrated microphone, the first thing you do is enter the file into the processor, typically, a computer which interfaces with the electronics.

Anthem offers calibrated microphones starting with the entry level $1000 AVR. Audyessy-enabled AVRs products can be attached to an optional calibrated microphone, but only a custom installer can access it, which is a major downside. Both Anthem and Audyessy, with the calibrated microphone, use a computer that displays pre- and post-frequency response charts. The Anthem and Audyessy enabled products are AVRs with all the disadvantages to stereo users outlined above.